Luminescent rare-earth doped alkaline-earth fluorides have long been known, and have been employed for numerous purposes such as scintillation detectors and laser materials. CaF2 doped with such rare-earth species as Eu+3, Er+3, Tb+3 are well-known compositions. It is well-known that a rare-earth doped alkaline earth fluoride will exhibit luminescence when exposed to ultraviolet light.
Each rare-earth element when incorporated into an alkaline earth host lattice such as CaF2 exhibits a characteristic excitation spectrum; see, for example, FIG. 1 (101), and a characteristic emission or luminescence spectrum that depends upon the excitation wavelength employed; see, for example, FIG. 1 (102). The excitation spectrum is determined by monitoring the luminescence intensity at one wavelength while the specimen is illuminated over a range of wavelengths. The luminescence spectrum is determined by illuminating the specimen at a single wavelength corresponding to a peak in the excitation spectrum and determining the luminescence spectrum by scanning a detector over a range of wavelengths.
As shown in the FIGURE, each such spectrum consists of a plurality of peaks at different wavelengths of light. The wavelengths at which the peaks occur are characteristic of each rare-earth element. No two rare-earth elements exhibit the same excitation or emission spectra; that is, the peaks in their spectra do not in general arise at the same wavelengths. To obtain luminescence, the rare-earth element must be excited by a light source that emits light at a wavelength corresponding to the location of one of the peaks in the excitation spectrum. In general, the peaks in any one spectrum of rare-earth elements differ from one another in height or intensity, these differences in intensity being characteristic of the rare-earth element under particular conditions of measurement. These and related matters are all well-documented in the art. See for example, Martin et al., Atomic Energy Levels-the Rare-Earth Elements, U.S. Department of Commerce, National Bureau of Standards (1978).
It is known in the art that rare-earth-doped alkaline earth fluorides synthesized at temperatures below 100° C. exhibit a characteristic luminescence spectrum. See for example, Faulques et al., J. Fluor. 8 (4), pp. 283-287 (1998), discloses that room-temperature synthesized rare-earth-doped fluorides will undergo changes in luminescence spectrum when heated to high temperatures for sufficiently long durations.
As disclosed in the art, considerable effort is directed towards developing compositions comprising luminescent rare-earth doped fluorides for use as identifying markers on commercial goods, including packages, manufactured articles, and even money. One high value application contemplated is in the area of “security markers” or anti-counterfeiting marks on goods. The idea is to place an identifying mark on a manufactured article which will attest to its authenticity. The mark is ideally invisible until inquiry is made using a particular wavelength of light which then stimulates the rare-earth doped fluoride to luminesce with a characteristic spectrum.
Security marks known in the art generally lack the complexity or encryption which would make them difficult to counterfeit. The present invention provides a family of novel rare-earth-doped alkaline earth fluorides, and a process for preparing them, that are characterized by continuously variable luminescence peak intensity ratios, making it extraordinarily difficult to counterfeit.